Investigation of the Spatial and Temporal Variations of the Seasonally Frozen Ground in the Contiguous United States

P.I./Institution: Tingjun Zhang, Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder

Co-P.I.(s)/Instituions: Dr. Christoph Oelke, Dr. Richard Armstrong, Dr. Martyn Clark Institution: Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder

Summarize in the two to four sentences what activities will be (are being) carried out in the project: A new frozen soil algorithm will be developed and validated using ground-based measurements. Using the validated frozen soil algorithm we will investigate the variations of soil temperature and seasonally frozen ground in the contiguous United States for the period from 1978 through present with resolution of 2 km. Combined with other environmental data, we will investigate seasonal and inter-annual changes of soil temperature and seasonally frozen ground and their environmental controls. We will produce a comprehensive soil temperature and frozen soil data product to be available to the scientific community.

Project Abstract: Soil temperature and freeze/thaw status have a significant impact on plant growth, carbon exchange, ecosystem diversity and productivity, surface energy and moisture balance in the climate system. Frozen soils have a significant impact on land surface and subsurface hydrology due primarily to the reduction of the hydraulic conductivity. Much of the land surface in the USA experiences seasonal freezing up to several months in duration. It is clear that understanding the dynamics of the ground thermal regime and the seasonal freeze/thaw processes, the seasonal and interannual variations in timing, duration, area extent and thickness, and the response to climatic change are important aspects of estimating ecosystem productivity and net ecosystem exchange of carbon and predicting the future changes in climate. We propose to investigate historical variations of daily soil temperature and frozen ground (timing and duration, daily thickness and areal extent over the study area) through passive microwave remote sensing (1978-present) over snow-free land area with a resolution of 25 km and numerical modeling (1978-present) with a resolution of 1 to 2 km over the entire proposed study area. This information is critical for regional extrapolation of net ecosystem exchange of CO2 from the tower footprints to large spatial scales. Timing of the thaw onset of seasonally frozen ground may be one of the key factors controlling the spring-time initiation of net carbon uptake by plants. During the past two years, a combined frozen soil algorithm has been developed, validated, and calibrated using ground-based measurements. The frozen soil algorithm has been used to detect the near-surface soil freeze/thaw cycle in the contiguous United States from 1987-2002. Results indicate that up to 70% of the land surface experience seasonal freeze/thaw processes over the study area. There are substantial seasonal and inter-annual variations in timing, duration, number of days, area extent and thickness of seasonally frozen soils.

Summary (200 words or less), in “layman’s term,” of what will be (is being) done and why: Soil temperature and freeze/thaw status have a significant influence on plant growth, carbon exchange and climate through surface energy balance, and surface and subsurface hydrology. Much of the land area in the USA experiences seasonal freezing each winter. This project intends to investigate seasonal and inter-annual variations of soil temperature and seasonally frozen ground in the USA. The results of this study will provide key information for the study of plant growth and carbon exchange, and will provide input and validation data for ecosystem and climate models as well as information for policy makers. We will use both satellite remote sensing data and numerical modeling to accomplish the proposed research.

How will the project contribute to one or more of the critical questions relevant to society, to the Regional Center’s strategic vision and goals, and the to the global change community? The proposed research directly addresses all three questions identified under the NIGEC “The AmeriFlux Network of Whole Ecosystem Flux Sites''. Soil temperature and soil freeze/thaw cycle are the key environmental parameters controlling CO2 exchange. Changes in soil temperature and frozen ground are also indicators and integrators of climate change.

For third-year continuation proposals, what are the “next-step” strategic questions to be addressed? During the year 3, we will Using the compiled data products and the validated frozen soil algorithm, we will: (i) complete all computations using the compiled data and the validated frozen soil algorithm for the period from 1978 through 2003; (ii) analyze the outputs from the frozen soil algorithm to reveal the climatology and the seasonal and inter-annual changes of area extent, timing, duration, number of days, frequency, and thickness of the seasonally frozen ground in the contiguous United States; Present new findings to scientific community and continue to prepare and submit manuscripts to peer reviewed journals for publications; Finalize the data products for archive and distribution.